Remove Toolchain::get_dependencies extra entries (#26)

* Remove Toolchain::get_dependencies extra entries

I'm using the `icx` compiler from OneAPI to compile OpenMP offload code. The following command used to query dependencies:
`icx -M -O3 -g -fPIC -Wall -std=c99 -qopenmp -fopenmp-targets=spir64 /home/u83311/tmp/devito-jitcache-uid85311/260030861f24c1e2bc347afa986d9632be371ca9.c`
returns the expected headers, as well as lines such as:
`# __CLANG_OFFLOAD_BUNDLE____START__ openmp-spir64`

which then later lead to issues because these are not files. I am not sure, what would be the ideal solution, perhaps filter out lines beginning with `#`?

* simplify line length check

Co-authored-by: Andreas Klöckner <inform@tiker.net>

* need brackets

* Linebreak and whitespace

Co-authored-by: Andreas Klöckner <inform@tiker.net>